Cast iron containing niobium and component

ABSTRACT

Known cast iron alloys have use limits in respect of the temperature. An alloy (in a weight percentage) including silicon 2.0%-4.5%, carbon 2.9%-4.0%, niobium 0.05%-0.7%, molybdenum 0.3%-1.5%, optionally cobalt 0.1%-2.0%, manganese≦0.3%, nickel≦0.5%, magnesium≦0.07%, phosphorus≦0.05%, sulphur≦0.012%, chromium≦0.1%, antimony≦0.004%, and, iron, is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2012/054941 filed Mar. 21, 2012 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the European Patent Office application No. 11162635.4 EP filed Apr. 15, 2011, the entire contents of which is hereby incorporated herein by reference.

FIELD OF INVENTION

The invention relates to a cast iron containing niobium as claimed in the claims and to a component as claimed in the claims.

BACKGROUND OF INVENTION

The known cast iron alloys now employed (so-called GJS alloys: nodular cast iron) primarily use silicon and molybdenum to increase the creep strength, scaling resistance and LCF behavior. Over time, however, these elements lead to a significant decrease in the toughness.

Molybdenum furthermore exhibits a very high susceptibility to segregation.

SUMMARY OF INVENTION

It is therefore an object of the invention to specify an alloy and a component, which overcome the aforementioned disadvantages and have better mechanical strengths over the service life.

The object is achieved by an alloy as claimed in the claims and a component as claimed in the claims.

The dependent claims list further advantageous measures which are advantageously combined with one another in any desired way.

The invention consists in the fact that cobalt and/or niobium can partially replace molybdenum. The working limitations presented by the previous GJS alloy can therefore be overcome.

The iron-based alloy according to the invention has high elongations for the application field in the temperature range of 450° C.-550° C., and has the following composition (in % by weight):

silicon (Si) 2.0%-4.5%, in particular 2.3%-3.9%, carbon (C) 2.9%-4.0%, in particular 3.2%-3.7%, niobium (Nb) 0.05%-0.7%, in particular 0.05%-0.6%, very particularly 0.1% to 0.7%, molybdenum (Mo) 0.3%-1.5%, in particular 0.4%-1.0%, very particularly 0.5%, optionally cobalt (Co) 0.1%-2.0%, in particular 0.1%-1.0%, manganese (Mn) ≦0.3%, in particular 0.15-0.30%, nickel (Ni) ≦0.5%, in particular ≦0.3%, magnesium (Mg) ≦0.07%, in particular at least 0.03%, very particularly 0.03%-0.06%, phosphorus (P) ≦0.05%, in particular 0.02%-0.035%, sulfur (S) ≦0.012%, in particular ≦0.005%, very particularly between 0.003% and 0.012%, chromium (Cr) ≦0.1%, in particular ≦0.05%, antimony (Sb) ≦0.004%, in particular ≦0.003%, iron (Fe), in particular remainder iron.

Advantageously, the proportion of silicon, cobalt, niobium and molybdenum is ≦7.5% by weight, in particular ≦6.5% by weight.

Even small proportions of cobalt and/or niobium and molybdenum improve the mechanical characteristics.

Niobium improves the endurance strength with a constantly high LCF strength and good toughness.

By the precipitation of finely distributed Nb carbides, niobium brings about a higher high-temperature strength, as a result of which the working limitations are shifted to high temperatures.

Cobalt brings about a solid solution solidification, which has a positive effect on the properties of the alloy at high temperatures and given low stresses.

The addition of molybdenum to the alloy (preferably 0.4%-1.0%) has a positive influence on the high-temperature strength (Rp0.2 and Rm in the elevated temperature range) and the endurance behavior (creep strength).

Preferably, the proportion of cobalt in the alloy lies between 0.5% by weight and 1.5% by weight.

Advantageous mechanical values are achieved for the alloy respectively when the cobalt content is 0.1% by weight to 1.0% by weight cobalt.

Magnesium obtains the nodular formation of the graphite and magnesium is preferably present in an amount of at least 0.03% by weight, at most 0.07% by weight.

Depending on the application, chromium (Cr) is preferably present in an amount of at least 0.01% by weight, but at most 0.05% by weight, and this increases the oxidation resistance.

The alloy may comprise further elements.

The alloy optionally contains small minimum admixtures of

phosphorus (P)  0.05% by weight, sulfur (S) 0.001% by weight, magnesium (Mg)  0.01% by weight, antimony (Sb), cerium (Ce),

which have a positive influence on the castability and/or the formation of the nodular graphite, but also must not be excessively high since otherwise the negative influences prevail.

Furthermore, there is preferably no chromium (Cr) in the alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be explained in more detail with reference to the following figures, in which:

FIG. 1 shows a steam turbine,

FIG. 2 shows a gas turbine.

DETAILED DESCRIPTION OF INVENTION

The component with the alloy has an optimal ferritic microstructure with nodular graphite.

The table shows exemplary alloys according to the invention which have improved mechanical properties.

C Si Mo Co Nb Mg Mn P S Sb 1 3.2 3.5 0.5 0 0.5 0.04 0.2 0.03 0.005 0.0009 2 3.3 3.6 0.5 0 0.1 0.05 0.2 0.03 0.005 0.0003 3 3.7 2.7 1.0 0.9 0.4 0.05 0.2 0.03 0.005 0.0004 4 3.5 2.4 1.0 0 0.5 0.06 0.3 0.03 0.004 0.0002 5 2.3 3.9 0.5 0 0.4 0.03 0.3 0.03 0.007 0.0030 6 3.3 3.4 0.5 1.0 0.5 0.04 0.2 0.02 0.005 0.0030 7 3.3 3.4 0.5 0.5 0.5 0.04 0.2 0.02 0.005 0.0039 8 3.0 3.3 0.4 0 0.2 0.05 0.2 0.03 0.004 0.0014

The alloy preferably contains no vanadium (V) and/or titanium (Ti) and/or tantalum (Ta) and/or copper (Cu).

The ratio of C and Si should give an almost-eutectic composition, i.e. should correspond to a carbon equivalent CE of between 4.1% and 4.4%,

$\left( {{CE} = {{\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} C} + \frac{{\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {Si}} + {\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} P}}{3}}} \right).$

FIG. 1 shows a steam turbine 300, 303 having a turbine shaft 309 extending along an axis of rotation 306.

The steam turbine comprises a high-pressure turbine part 300 and a medium-pressure turbine part 303, each with an inner housing 312 and an outer housing 315 enclosing the latter. The high-pressure turbine part 300 is, for example, configured in pot design. The medium-pressure turbine part 303 is, for example, configured to be twin-streamed. It is likewise possible for the medium-pressure turbine part 303 to be configured to be single-streamed.

A bearing 318 is arranged along the axis of rotation 306 between the high-pressure turbine part 300 and the medium-pressure turbine part 303, the turbine shaft 309 comprising a bearing region 321 in the bearing 318. The turbine shaft 309 is mounted on a further bearing 324 beside the high-pressure turbine part 300. In the region of this bearing 324, the high-pressure turbine part 300 comprises a shaft seal 345. The turbine shaft 309 is sealed relative to the outer housing 315 of the medium-pressure turbine part 303 by two further shaft seals 345. Between a high-pressure steam intake region 348 and a steam outlet region 351, the turbine shaft 309 in the high-pressure turbine part 300 comprises the high-pressure rotor blading 357. With the associated rotor blades (not shown in more detail), this high-pressure rotor blading 357 constitutes a first blading region 360.

The medium-pressure turbine part 303 comprises a central steam intake region 333. Associated with the steam intake region 333, the turbine shaft 309 comprises a radially symmetric shaft shield 363, a cover plate, on the one hand to divide the steam flow into the two streams of the medium-pressure turbine part 303 and also to prevent direct contact of the hot steam with the turbine shaft 309. In the medium-pressure turbine part 303, the turbine shaft 309 comprises a second blading region 366 with the medium-pressure rotor blades 354. The hot steam flowing through the second blading region 366 flows from the medium-pressure turbine part 303 out of a discharge port 369 to a low-pressure turbine part (not shown) connected downstream in terms of flow technology.

The turbine shaft 309 is composed for example of two turbine shaft parts 309 a and 309 b, which are connected firmly to one another in the region of the bearing 318. Each turbine shaft part 309 a, 309 b comprises a cooling line 372 formed as a central bore 372 a along the axis of rotation 306. The cooling line 372 is connected to the steam outlet region 351 via a feed line 375 comprising a radial bore 375 a. In the medium-pressure turbine part 303, the coolant line 372 is connected to a cavity (not shown in more detail) below the shaft shield. The feed lines 375 are configured as a radial bore 375 a, so that “cold” steam from the high-pressure turbine part 300 can flow into the central bore 372 a. Via the discharge line 372 also formed in particular as a radially directed bore 375 a, the steam passes through the bearing region 321 into the medium-pressure turbine part 303 and there onto the lateral surface 330 of the turbine shaft 309 in the steam intake region 333. The steam flowing through the cooling line is at a much lower temperature than the temporarily superheated steam flowing into the steam intake region 333, so as to ensure effective cooling of the first rotor blade row 342 of the medium-pressure turbine part 303 and the lateral surface 330 in the region of this rotor blade row 342.

FIG. 2 shows, by way of example, a partial longitudinal section through a gas turbine 100.

In the interior, the gas turbine 100 has a rotor 103 with a shaft 101 which is mounted such that it can rotate about an axis of rotation 102 and is also referred to as the turbine rotor.

An intake housing 104, a compressor 105, a, for example, toroidal combustion chamber 110, in particular an annular combustion chamber, with a plurality of coaxially arranged burners 107, a turbine 108 and the exhaust-gas housing 109 follow one another along the rotor 103.

The annular combustion chamber 110 is in communication with a, for example, annular hot-gas passage 111, where, by way of example, four successive turbine stages 112 form the turbine 108.

Each turbine stage 112 is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium 113, in the hot-gas passage 111 a row of guide vanes 115 is followed by a row 125 formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143, whereas the rotor blades 120 of a row 125 are fitted to the rotor 103 for example by means of a turbine disk 133.

A generator or a working machine (not shown) is coupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air 135 through the intake housing 104 and compresses it. The compressed air provided at the turbine-side end of the compressor 105 is passed to the burners 107, where it is mixed with a fuel. The mix is then burnt in the combustion chamber 110, forming the working medium 113. From there, the working medium 113 flows along the hot-gas passage 111 past the guide vanes 130 and the rotor blades 120. The working medium 113 is expanded at the rotor blades 120, transferring its momentum, so that the rotor blades 120 drive the rotor 103 and the latter in turn drives the generator coupled to it.

While the gas turbine 100 is operating, the components which are exposed to the hot working medium 113 are subject to thermal stresses. The guide vanes 130 and rotor blades 120 of the first turbine stage 112, as seen in the direction of flow of the working medium 113, together with the heat shield elements which line the annular combustion chamber 110, are subject to the highest thermal stresses.

To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant.

Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure).

By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane 120, 130 and components of the combustion chamber 110.

Superalloys of this type are known, for example, from EP 1 204 776 B 1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blades or vanes 120, 130 may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Alloys of this type are known from EP 0 486 489 B 1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

It is also possible for a thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO₂, Y₂O₃—ZrO₂, i.e. it is unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide.

Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).

The guide vane 130 has a guide vane root (not shown here), which faces the inner housing 138 of the turbine 108, and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor 103 and is fixed to a securing ring 140 of the stator 143. 

1-18. (canceled)
 19. An iron-based alloy comprising (in % by weight): silicon 2.0%-4.5%; carbon 2.9%-4.0%; and molybdenum 0.3%-1.5%;


20. The alloy as claimed in claim 19, further comprising: cobalt 0.1%-2.0%, manganese  ≦0.3%, nickel  ≦0.5%, magnesium  ≦0.07%, phosphorus  ≦0.05%, sulfur ≦0.012%, chromium  ≦0.1%, antimony ≦0.004%, and iron,

wherein, for carbon, silicon and phosphorus, has a CE equivalent of 4.1% to 4.4%, and wherein ${CE} = {{\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} C} + \frac{{\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {Si}} + {\% \mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} P}}{3}}$ holds true.
 21. The alloy as claimed in claim 19, further comprising 0.1% by weight-0.2% by weight niobium.
 22. The alloy as claimed in claim 19, further comprising 0.4% by weight-0.6% by weight niobium.
 23. The alloy as claimed in claim 20, wherein the alloy does not comprise cobalt.
 24. The alloy as claimed in claim 20, wherein the alloy comprises 0.4% by weight to 0.6% by weight cobalt.
 25. The alloy as claimed in claim 20, wherein the alloy comprises 0.9% by weight to 1.0% by weight cobalt.
 26. The alloy as claimed in claim 20, wherein the alloy comprises 1% by weight to 2% by weight cobalt.
 27. The alloy as claimed in claim 20, wherein the alloy comprises 0.1% by weight cobalt.
 28. The alloy as claimed in claim 21, wherein a proportion of silicon, cobalt, molybdenum and niobium is less than 6.5% by weight of the alloy.
 29. The alloy as claimed in claim 21, wherein a proportion of molybdenum and niobium does not exceed 1.5% by weight of the alloy.
 30. The alloy as claimed in claim 19, wherein the alloy comprises 2.0% by weight-3.0% by weight silicon.
 31. The alloy as claimed in claim 19, wherein the alloy comprises 3.0% by weight-4.5% by weight silicon.
 32. The alloy as claimed in claim 19, wherein the alloy contains no nickel except as a possible impurity.
 33. The alloy as claimed in claim 20, wherein the alloy comprises at least 0.01% by weight nickel.
 34. The alloy as claimed in claim 19, wherein the alloy comprises 2.5% by weight to 3.7% by weight carbon.
 35. The alloy as claimed in claim 19, wherein the alloy comprises no chromium except as a possible impurity.
 36. The alloy as claimed in claim 20, wherein the alloy comprises at least 0.01% by weight chromium.
 37. A component, consisting of: an alloy as claimed in claim 19, wherein the alloy is a housing part of a steam turbine or of a gas turbine. 